Metallothermal reactions and mixtures for carrying out the reactions have been known for more than 100 years. The metallothermal reaction is based on the reduction of a metal oxide by a metal which is more electronegative than the metal of the metal oxide. The reaction, after local ignition of the reaction mixture, propagates with the evolution of heat more or less rapidly through the metallothermal reaction mixture. At the same time, the more electronegative metal is oxidized and floats as a molten slag, while the melt of the more electropositive metal separates from the slag and collects in the lower part of the reaction vessel. Technically, aluminothermal and calciothermal reaction mixtures have proven to be successful, in particular, for the production of iron and steel melts as well as of carbon-free metals and special alloys.
To begin with, it is difficult to carry out the metallothermal, highly exothermic reaction in a technically manageable form. It is important in this regard to start the reaction by punctiform ignition with an easily ignitable pyrophoric mass or by a magnesium foil, as was described for the first time in 1895 in the German patent 96 317.
To achieve a steel melt of a particular composition, alloying elements must be added in different form and composition to the iron oxide/aluminum mixture. Alloying partners can be added to the reaction mixture as metals in grit form or in the form of their oxides or other chemical compounds. It is possible to add carbon in free form or in the form of carbide, in order to bring about a carburization of the aluminothermally produced steel. Materials to attenuate the exothermic reaction, in the form of, for example, grey cast iron waste pieces, structural steel stamping waste, etc. can be added to the reaction mixture.
In all these cases, however, to achieve a reproducible reaction product, it is necessary that the aluminothermal or generally the metallothermal reaction proceeds as uniformly as possible and that this uniform course of the reaction is reproducible. If the reaction proceeds at different rates, the consumption of the alloy-forming components can proceed differently. This leads to alloys of different composition and, with that, also of different properties. If the reaction is carried out in a casting crucible, the bottom opening of which is sealed by a meltable seal, as described, for example, in the German patent 32 11 83 1, the melting of the seal should take place after a precisely predetermined time interval following ignition of the mixture in order to ensure that the reaction has come to an end and the slag has separated completely from the metal melt. If the seal melts too early, liquid slag particles, which have not yet been separated, can be carried along by the metal melt flowing out. If the seal melts too late, the melt may have already cooled too much and, with that, assume a state which is undesirable for certain technical processes.
Attempts have been made to improve the reproducibility of the metallothermal reactions by optimizing tile reaction crucible with respect to its shape (pointed cone with a different angle of slope), its size, lining, cover, etc. Certain improvements have been achieved. To a large extent, however, the manual dexterity and experience of the persons entrusted with carrying out the reaction affected the outcome of the reaction and its reproducibility.